The present invention relates to a method for producing a record copy disc, particularly a disc having a high groove density.
Disc recordings are usually produced by cutting a spiral-shaped groove modulated with the information into a rotating body called an original. The material into which the groove is cut may be a lacquer, as is often used in the manufacture of phonograph records, or a soft, very homogeneous metal, e.g. copper. From the pattern cut into the original a reproduction called a master is made by electrochemical means. This master is a negative of the disc copies to be produced.
After production of the master, the center of the spiral groove transferred to the master is first determined and marked by measuring, with the aid of optical enlargers, the distance of one groove ring from various points on the periphery. The disc is adjusted so that this distance is the same everywhere. Special optical devices are available for this purpose. When this method is used, a maximum eccentricity of, for example, 0.2 mm can be maintained, which is sufficient for conventional phonograph records.
However, for rigid discs employed to provide high storage, or groove, densities the eccentricity of the center of the groove being played back must not exceed, for example, 0.02 mm.